In a tire, running causes periodical distortion in a rubber portion. A part of the energy thereof is converted into heat, and the tire produces heat. In particular, when the tire keeps running with decreased air pressure, each strain becomes large, and much of heat generates. When a rubber temperature of the tire exceeds a certain value, the rubber fracture begins. Therefore, it is useful in enhancing durability of the tire to keep the temperature of the running tire down.
And, in recent years, a run flat tire is prevalent for the purpose of enhancing convenience and safety and enlarging a vehicle interior space. It is well known that the run flat tire is of a side-reinforced type in which an inside of each sidewall portion is provided with a side reinforcing rubber layer having a substantially crescent cross-sectional shape (see the following patent document 1, for example). In this run flat tire, even at a time of puncture for example, the side reinforcing rubber layer receives a tire load instead of air pressure, and eventually a deflection of the sidewall portion is limited. Therefore, even when the tire is punctured the run flat tire is capable of running ongoingly about from 50 to 100 km at a speed of from 60 to 80 km/h, for example, (hereinafter, such running is so-called “run-flat running”).
However, although it is the run flat tire, the side reinforcing rubber layer generates heat in proportion to running distance during the run-flat running with decreased air pressure. And when the running distance becomes over a limiting value, the side reinforcing rubber layer breaks because of heat degradation.    Patent document 1: Japanese unexamined Patent Application Publication No. 2006-182318.